Sunglass materials have traditionally been designed to provide a desired tint and significant advances have been made in the prior art in controlling tint and providing compositions that provide green, grey and brown tints. New applications, however, are placing increasingly stringent demands on the performance of sunglass materials. Of particular recent is the desire for sunglass materials that are capable of filtering ultraviolet (UV) and infrared radiation (IR) with high efficiency. Sunglass materials must, however, provide UV and IR protection while preserving tint and insuring faithful recognition of color to insure compliance with vehicular safety standards that require correct identification of traffic signals by motorists wearing sunglasses.
Tint is normally controlled by adding one or more colorants to a base glass composition. The base glass composition is typically a silica or modified silica composition, such as aluminosilicate glass, borosilicate glass, or aluminoborosilicate glass with alkali or alkaline earth modifiers. Common colorants include transition metal oxides and rare earth oxides. The component oxides are batched in appropriate amounts, melted, homogenized and cooled to form a glass composition having a desired tint. UV-absorbing modifiers, such as iron oxide or vanadium oxide, can be included in the composition to improve UV absorption. Iron oxide is also desirable for controlling IR absorption because of the mixed valent nature of Fe. Fe can be present in oxides as the ferric ion (Fe3+), the ferrous ion (Fe2+), or a combination thereof. The ferric ion provides efficient UV absorption and the ferrous ion provides efficient IR absorption. By properly balancing the relative amounts of ferric ion and ferrous ion in the glass composition, a composition having advantageous absorption in both the UV and IR can be achieved
The drawback in using iron oxide to achieve a glass composition that efficiently filters UV and IR radiation is that iron oxide also influences transmission in the visible. A typical iron-containing commercial glass composition that absorbs efficiently in the UV and IR has a visible transmittance of about 30%. While 30% visible transmittance is acceptable for many applications of sunglasses, it is too low to permit modification of tint through inclusion of colorants that absorb in the visible. Additional colorants further reduce the visible transmittance of the sunglass material to a level that makes the sunglasses unacceptable for many applications. The multivalent nature of the transition metal and rare earth ions of many oxide colorants also influences the equilibrium balance between ferric and ferrous ions needed to achieve the desired strength of UV and IR absorption.
Due to the lack of flexibility in adding colorants, the range of tints available for sunglasses with efficient filtering of both UV and IR wavelengths is limited. Most notably, it is difficult to obtain sunglasses with a brown tint and adequate visible transmittance that absorb strongly in the UV and IR. U.S. Pat. No. 6,255,238 describes one approach for obtaining brown tint while maintaining strong absorption in the UV and IR. The approach relies on using selenium oxide (SeO2) as a colorant to obtain brown coloration in sunglass materials that exhibit strong UV and IR absorption and adequate visible transmittance.
From a processing standpoint, however, use of SeO2 in glass fabrication is problematic because it has high volatility at the batch melt temperatures required for typical compositions. The high volatility makes it difficult to control the concentration and uniformity of the SeO2 content of the glasses. There is accordingly a need to develop sunglass materials having a brown tint along with satisfactory transmittance in the visible and satisfactory absorption in the UV and IR.